Tumor metastasis results from the spreading of a primary tumor and establishment of secondary tumors, mostly in vital organs such as the lung, liver, and brain. Tumor metastasis is responsible for the majority of cancer deaths, yet it remains the least understood stage of cancer.Metastatic tumor cells acquire new biological features allowing them to migrate, invade normal tissues, escape the immune system control, and develop drug resistance. Many mechanisms have been shown to play an important role in the metastatic process, particularly through the tumor microenvironment. Many factors present in the tumor microenvironment have been identified and demonstrated to play either a pro-or anti-metastatic role, allowing the development of new targeted therapies.However, despite an armamentarium of treatments, patients remain at a high risk for metastasis. Better understanding of the mechanisms leading to cancer metastasis is crucial for the development of novel and more efficacious therapies.Currently many research areas are involved in the identification of novel biological markers and therapeutic targets. Modern approaches including genomics, molecular biology, immunology, imaging and computational biology, are carried out in cancer research, leading to the development of novel therapeutic options. Many therapeutic options based on the manipulation or modulation of the immune system have shown unprecedented results and have provided new hope to cancer patients. The introduction of targeted and immune-based therapies have significantly improved the cancer survival rate. In addition to discussing relevant options of cancer treatment and ongoing clinical trials, recent breakthroughs in cancer development and metastasis are provided in this book.